$ \left(\dfrac{5}{2}\right)^{-4}$
Answer: $= \left(\dfrac{2}{5}\right)^{4}$ $= \left(\dfrac{2}{5}\right)\cdot\left(\dfrac{2}{5}\right)\cdot \left(\dfrac{2}{5}\right)\cdot \left(\dfrac{2}{5}\right)$ $= \dfrac{4}{25}\cdot\left(\dfrac{2}{5}\right)\cdot \left(\dfrac{2}{5}\right)$ $= \dfrac{8}{125}\cdot\left(\dfrac{2}{5}\right)$ $= \dfrac{16}{625}$